Semiconductor memory devices, such as dynamic random access memories (DRAMs) and nonvolatile memories, store data, read stored data and/or erase stored data. A unit cell of a DRAM generally consists of one metal oxide semiconductor (MOS) transistor and one capacitor. As semiconductor memory devices become more highly integrated, the size of the memory device is scaled down and, therefore, the capacitor area is reduced. Thus, it may be difficult to obtain a sufficient capacitance.